The present invention relates to a bent glass sheet suitably used for a vehicle window, especially for a vehicle window having a mechanism to move the windowpane along the surface direction.
Typical windows for front doors of automobiles are opened and closed by moving the windowpanes along the surface direction of the vehicle bodies. In such a case, windowpanes are shaped in general to be flat or cylindrical. A cylindrical glass sheet slides along a direction with a predetermined curvature radius and reciprocates between the inside of the door and the opening.
Recently, glass sheets bent both in the sliding direction and in the longitudinal direction of the vehicle are required to improve consistency with the vehicle""s design. Such a bent glass sheet is often called a glass sheet having a xe2x80x9ccomplex curvedxe2x80x9d surface or a xe2x80x9ctwo directionalxe2x80x9d bent glass sheet.
A surface of such a bent glass sheet is required to be bent with high accuracy along a predetermined curved surface. When the surface has a point at which the curvature varies, reflected images on the glass surface will be distorted in the vicinity of the point. Especially, such a distortion of reflected images should be removed from windowpanes composing a sideline of an automobile.
A bent glass for a rear window or the like of such an automobile is not bent to have a constant curvature. Therefore, even if some regions of the bent glass sheet have curvatures different from a predetermined curvature, the influence is difficult to identify visually. On the other hand, since a glass sheet that is bent in two directions to have identical curvatures will provide extremely natural reflected images as a whole, even partial changes in the curvature and defects on the surface will be conspicuous.
When the glass sheet is used for a door window, preferably the windowpane slides while it is maintained at a certain position as much as possible in an opening (slit) formed at the upper end of the door housing. A wider slit for a margin may permit rainwater or dust to enter the door housing easily. Weatherstrips are provided for the slit in order to prevent passage of foreign objects. However, if the windowpane is leaned excessively in one direction, friction between the weatherstrips and the windowpane becomes unduly large, or a space is created between these them.
Because of these conditions, if a glass sheet for a vehicle window is bent in two directions, it should be processed with higher accuracy when compared to a bent glass sheet with a further complicated shape.
Various methods to bend glass sheets have been proposed. Surprisingly however, two-directional bent glass sheets with accuracy to respond to the demands of the market have not been manufactured, even though such glass sheets are required from a design viewpoint. This results from the limited accuracy in the conventional bending methods.
A typical method to bend a glass sheet is press molding. This method is suitable for mass-production of bent glass sheets with a complicated shape that are used for rear windows or the like of automobiles. However, in a glass sheet 503 that is pressed while being supported with a ring mold 501 and being attracted by a mold 502, a central portion 504 hangs down slightly (FIG. 33B) before it is cooled. If the degree of deformation caused by gravity could be expected, the shape of the mold might be adjusted finely. However, since there are various factors determining the deformation, precise expectation of the deformation degree is substantially impossible. Similarly, in press-molding using tongs, the deformation degree is difficult to expect. In another method, a mold is arranged entirely along both surfaces of a glass sheet to be bent before cooling the bent glass sheet while keeping the glass sheet along the mold. This method also might be available. However, since a great internal stress remains in the glass sheet, the bent glass sheet may be deformed when it is released from the mold. In conclusion, problems should be solved to manufacture with accuracy a glass sheet that is bent in two directions by press-molding.
U.S. Pat. No. 4,123,246 and JP-A-3-174334 propose a method to heat glass sheets while conveying them by rollers in order to bend the glass sheets along the rollers. However, the glass sheets will be bent intermittently if they are bent by using rollers arranged above and below the conveying passage. More specifically, the glass sheets will be bent and folded in part while they are bridged between pairs of rollers, especially in initial stages of the bending process. When the glass sheet is bent also in a direction perpendicular to the conveying direction, the rollers should be bent before being arranged. It is difficult to bend the respective rollers arranged along the conveying direction with high accuracy and rotating the rollers in that state. Bending glass sheets by sandwiching them with rollers is excellent in view of mass production but it cannot provide sufficient processing accuracy.
JP-A-5-9037 proposes a method of conveying glass sheets while supporting the same sheets with a gas spouting upwards from a hearth bed arranged below. The gas heats the glass sheet at the same time, and the sheet sags down along the hearth bed. In this manner, defects due to roller xe2x80x9cbending/foldingxe2x80x9d or due to contact with rollers can be prevented. However, the passage to convey glass sheets should be inclined to bend the glass sheet in two directions and thus, conveying glass sheets while keeping a stable orientation is difficult. When the glass sheet cannot be oriented stably, accuracy in forming the glass sheet also will deteriorate. The method of conveying and bending glass sheets by spouting a gas is suitable for providing a simpler form, but the process accuracy is sacrificed when a glass sheet is bent in two directions.
As mentioned above, it has not been considered in any conventional bending methods that a glass sheet is cooled (quenched or annealed) after a bending process while keeping the bent shape accurately, nor have any ideas relating to such methods been disclosed.
Because of the above-mentioned reasons, no methods to produce glass sheets bent in two directions with accuracy have been established. In the above references, manufacturing of glass sheets bent in two directions is attempted. The glass sheets according to the references may be bent in two directions as a whole, but some regions thereof are not bent as desired when observed in detail, because of limitations resulting from the methods themselves. As a result, bent glass sheets arranged in the sides of an automobile such as a door glass are shaped in a simple manner actually.
JP-A-11-500796 proposes to use door glass made of a glass sheet that is bent to have a more complicated shape than a glass sheet bent in two directions. This reference discloses a mechanism to lift a bent glass sheet of an imaginary barrel-shaped sleeve surface along the longitudinal direction of the vehicle. This mechanism allows a glass sheet of a barrel-shaped sleeve surface to lift while swiveling to maintain the lower side of the glass sheet in parallel. In such a bent glass sheet with complicated shape, distortion of reflected images would be less distinguishable.
However, a complicated lifting mechanism should be provided to a space formed inside the door (door housing) in order to lift the glass sheet. Such a complicated mechanism will cause troubles and raise costs. For swivel-lifting a glass sheet of a barrel-shaped sleeve surface, a guide rail that is bent three-dimensionally so as to describe a spiral is needed. If the guide rail has complicated curved lines, a wire is also bent at a point, and the durability of the wire can be negatively affected due to friction with the guide rail. In some cases, changing tension of the wire hinders smooth operation. Positions of the glass sheet at the door slit will also vary considerably.
For the above-mentioned reasons, the market""s needs for glass sheets that are bent accurately in two-directions are further increasing in spite of difficulties in production thereof.
The inventor started to improve the bending process. To bend a glass sheet accurately to provide a desired shape, preferably the entire surface of at least one side of a glass sheet is supported in bending the glass sheet. A mold (bending member) should be prepared for this purpose. However, the glass sheet will be deformed as mentioned above if it is lifted with a member such as a ring mold, or if the glass sheet is hung with tongs in pressing the glass sheet against the mold.
After keen examinations, the inventor successfully processed and bent a glass sheet with accuracy by pressing the glass sheet together with a heat-resistant belt against a mold, and prevented the glass sheet from being deformed even in a step of cooling thereof. The bent glass sheet of the present invention is not limited to what is bent with a belt, but such a bent glass sheet became feasible for the first time by using a belt well-fitting to the surface of the glass sheet and also by adapting the shape of the conveying passage to the shape of the bent glass sheet in a cooling step.
A bent glass sheet for a vehicle window according to the present invention is obtained by bending a flat glass sheet that is heated to a temperature between a strain point and a softening point of the glass sheet, and formed as follows. The bent glass sheet has a substantially uniform thickness and the main surfaces are curved surfaces. All points on the curved surface have a maximum curvature in the direction of one of the two tangent vectors (a first tangent vector) that are contacting the curved surface and crossing each other perpendicularly, while the same curved surface has a minimum curvature in the direction of the other tangent vector (a second tangent vector).
A bent glass sheet for a vehicle window according to the present invention is formed as follows.
A bent glass sheet according to the present invention has a substantially uniform thickness and the main surfaces are curved surfaces. All points on the curved surface have a maximum curvature in the direction of one of the two tangent vectors (a first tangent vector) that are contacting the curved surface and crossing each other perpendicularly, while the same curved surface has a minimum curvature in the direction of the other tangent vector (a second tangent vector).
The bent glass sheet according to the present invention is further characterized in that all the points have the substantially same maximum curvature. Moreover, this maximum curvature is substantially equal to a curvature at every point on a curved line formed by crossing this curved surface and a flat plane comprising a normal vector at one point on the curved surface and a tangent vector providing the maximum curvature at the one point. The minimum curvature is neither 0 (i.e., the glass sheet is not cylindrical) nor equal to the maximum curvature (i.e., the glass sheet is not spherical). Preferably, the curvature radius of the curved line is no less than 500 mm but less than 5000 mm.
In this specification, the term xe2x80x9cmain surfacesxe2x80x9d indicates a front/back surface pair of a glass sheet without the edge surface. The edge surface of the glass sheet includes a processed surface (polished surface) formed by grinding the edge of the glass sheet (i.e., the main surface does not include a polished surface).
Preferably in the bent glass sheet according to the present invention, the minimum curvature is substantially equal to a curvature at every point on a curved line formed by crossing this curved surface and a flat plane comprising a normal vector at one point on the curved surface and a tangent vector providing the minimum curvature at the one point. Preferably the curvature radius of the curved line is from 5000 mm to 50000 mm.
It is further preferable that the bent glass sheet according to the present invention is a tempered glass sheet toughened by heating and then quenching the glass sheet.
Thickness of the bent glass sheet is not specifically limited but preferably, it is from 2.3 mm to 5.0 mm.